Israel says it has refuted accusations of its responsibility for the deaths using shrapnel recovered from casualties and its timeline of events.

But evidence collected by Human Rights Watch indicated the civilians were killed within the time period of Israeli shelling of the beach, contrary to Israel's assertions, a statement from the group says.

During a meeting with the military investigator, Meir Klifi, Mr Garlasco says he put forward the possibility the blast was caused by the delayed explosion of an Israeli shell fired earlier that day.

The army fired more than 80 155mm shells in the area, Human Rights Watch says, and one of these may have got stuck in the sand waiting to be detonated.

Most victims of the blast were from one Palestinian family

Israeli investigators have suggested the blast could have been caused by a mine planted by Palestinian militants to deter Israeli commando raids.

The head of the Israeli military panel is reported to have told Mr Garlasco that the Palestinians had "no problem lying" about the incident, so he discounted their evidence.

"If the Israeli allegations of tampered evidence are to be believed, many Palestinians would have to have engaged in a massive and immediate conspiracy to falsify the data," said Mr Garlasco - a former Pentagon intelligence analyst.

"The conspirators - witnesses, victims, medical personnel and bomb disposal staff - would have had to falsify their testimony, amend digital and hand-written records, and dip shrapnel into a victim's blood," he said in a Human Rights Watch statement

"It beggars belief that such a huge conspiracy could be orchestrated so quickly."